The present invention relates to a ceramic heater formed of a high-temperature heating sintered body used in a glow plug and the like for ignition of various heaters and combustors and auxiliary starting of a diesel engine, and a method of manufacturing the same.
A glow plug is used as, e.g., an auxiliary heat source (preheat source) in starting a diesel engine. A known glow plug of this type conventionally, generally has a structure in which a metal heater made of Ni--Cr (nickel-chromium), Fe--Cr (iron-chromium), or the like is buried in a heat-resistant insulating powder such as MgO (magnesia), and the metal heater buried in the heat-resistant insulating powder is covered with a heat-resistant metal sheath.
In a glow plug of this type, the rise time to 800.degree. C. must be shortened, the peak and saturation temperatures must be set higher, and the afterglow time must be prolonged in order to meet today's exhaust gas regulations. To achieve these conditions, a refractory metal must be employed at the heater portion of the heating element. The sheath portion is made of not a metal but a ceramic usable at a high temperature.
For example, Japanese Patent Laid-Open No. 55-126989 discloses, as a ceramic heater used in the above-mentioned glow plug, the structure in which a refractory metal such as W (tungsten) is buried in a silicon nitride insulating ceramics excellent in oxidation resistance and thermal shock resistance.
In this ceramic heater, however, cracking, peeling, and the like occur during sintering, or the resistance of the heater portion becomes unstable because a reaction layer such as a tungsten silicide layer is formed between the metal and the insulating ceramics (particularly on the metal side). Owing to the manufacturing problems described above, the temperature cannot be increased so high in sintering the ceramics, and the ceramics is sintered using a hot pressing.
Even in the hot-press sintering, the maximum sintering temperature is 1,750.degree. C. at most. If the ceramics is sintered at a higher temperature, a reaction layer such as a tungsten silicide layer is confirmed to be formed between the metal and the insulating ceramics.
It is difficult to sinter silicon nitride-based ceramics at 1,750.degree. C. or less unless the amount of sintering aids is increased or low-melting sintering aids are selected. However, when the amount of sintering aids is increased, or the low-melting assistant is selected, high-temperature properties of the silicon nitride-based ceramics degrade. Further, a silicon carbide-based ceramics cannot absolutely be sintered at 1,750.degree. C. or less.
To prevent formation of a reaction layer such as the above tungsten silicide layer, Japanese Patent Laid-Open No. 61-179084 discloses the structure in which the surface of a resistance heating element made of W (tungsten) or Mo (molybdenum) is covered with a non-oxide ceramics.
However, this structure requires an additional step of covering the surface of the resistance heating element with the non-oxide ceramics, resulting in high manufacturing cost. Further, it is difficult that the surface of the coiled W is covered uniformly with a non-oxide ceramics.